In general, a power steering system that generates steering force for a vehicle is designed to use hydraulic pressure for smooth and rapid steering with a small force.
The power steering system has the advantage of steering a vehicle with a small force, selecting operational force regardless of the steering gear ratio, and preventing impact due to roughness of the road surface from being transmitted to the steering wheel by absorbing it.
Further, for example, motor driven steering wheel systems, such as EHPS (Electro-Hydraulic Power steering) or MDPS (Motor Driven Power Steering), which allow comport and smooth steering while traveling at a low speed, heavy steering with a good directional stability while traveling at a high speed, and rapid steering in an emergency by driving a motor using an ECU (Electronic Control Unit) depending on the traveling condition of a vehicle that is detected by a vehicle speed sensor and a steering torque sensor to provide optimum steering condition to the driver, has been used in recent years.
Furthermore, an AFS (Active Front Steering) control system of an active front wheel steering type is applied. The AFS, a single output steering system for double input (the input transmitted through the steering wheel from a driver and input from a motor), can increase or decrease the steering angle using an actuator as well as the steering wheel by the driver. Therefore, it improves steering response by reducing the steering ratio at a low speed and increasing the steering ratio at a high speed so that the driver can relatively stably drive a vehicle even at a high speed.
Further, in addition to achieving smaller or larger steering output than the force applied by the driver, the AFS can accomplish active control of a vehicle by integrally controlling with the cooperation of other systems such as ESC (Electronic Control Suspension System) or MDPS (Motor Driven Power Steering).
Considering the characteristics in operation, the AFS can be largely divided into an actuator that applies additional steering input other than the steering wheel from the driver and an AFS ECU that changes the steering output by controlling the actuator. The actuator, the most important part for the performance of the AFS, includes a common motor and a decelerator.
Further, since the AFS controls the steering input from the driver using power of the motor, it is possible to vary the configuration and operating and mounting ways of the actuator. Therefore, it is possible to reduce the entire configuration of the AFS as well as the size of the actuator itself.
However, since the decelerator of the actuator in the AFS receives both the input from the driver and the motor, the configuration of the decelerator becomes complicated and the size of the actuator increases. Therefore, the actuator has many limitations in mounting to the steering system, and even if it is mounted, it increases the entire size, such that it is difficult to accomplish a compact AFS.
Further, it is possible to manufacture the decelerator in a simple structure to achieve a compact AFS, for example, when the steering wheel input member of the decelerator and the AFS ECU input member are coaxially arranged and a harmonic gear-typed decelerator that can provide a high gear ratio in a small space is used, it is possible to reduce the size of the AFS.
However, the harmonic gear-typed decelerator requires high machining precision for the harmonic gear, such that it is difficult to manufacture the harmonic gear-typed decelerator at high manufacturing cost. Therefore, the harmonic gear cannot be easily used when it is needed.